Some of the language used in the book is decidedly ambitious but there are reasons for this.
Firstly, the vocabulary chosen seemed to be the best words for the (pleasurable) task in hand.
As importantly, in a short poem with a clear context, it is relatively straightforward to lead children to an understanding of the words and thereby extend their vocabulary. The meaning of a word like "preened" is soon established because of the evidence of vanity in the illustration as well as the couplet itself. Arguably, it is also onomatopoeic.Any undue sense of effort is lightened by the bathos of contrasting, colloquial expressions, such as "got stuck into the chocs". This also makes children laugh because they are acutely aware of minor hypocrisies in adult behaviour!
The very short form of rhyming couplets presents a clear model for children to emulate, particularly because there is a deep satisfaction in discovering an apt rhyme to lock your work into place. I enjoy it, I admit!
Phrases like "provoked some ugly scenes", "the daily grind", "went weak at the knees", "brought her close to the edge", "eating on the hoof" and so on are often quite useful for making children's writing more colourful and introducing humour.